Arrow spine question for long draw archer - 33.5 in Draw length

Jg48

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Hey guys

Shooting a Mathews atlas at 74 lbs, 33.5 inch draw length (I’m 6’8” with 7ft wing span)

Looking at kinetic khaos 250 or 200?? Arrows will be uncut at 32in

The arrow charts only look at arrow length and poundage, does having a longer draw length than arrow length have an impact on when choosing a spine? Thanks!
 
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Not really. It plays in a little, but it's mostly the arrow length, the tip weight, and the # of pounds pushing it. The ibo of the bow comes into play since that pertains to how aggressive the cam is. The Atlas is a pretty slow cam compared to a lot of other models. It really doesn't push a spine too hard.

Remember with the ballistic collar you loose a little length, can get close to your rest.

I also pretty much always go a spine heavier from what the charts say with a hunting arrow, tho like I said the Atlas doesn't push a shaft as hard so you might be ok, you might not have options either.

Pm me your details, I can send you some numbers on where you will be. I don't like putting info from a paid service out in the open.
 

5MilesBack

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The arrow charts only look at arrow length and poundage, does having a longer draw length than arrow length have an impact on when choosing a spine? Thanks!
I would expect it to. Consider a guy with a 29" draw shooting 30" arrows versus a guy with a 33" draw shooting 30" arrows........all else equal. That's a 40fps difference in arrow speed, therefore more energy launching that arrow. That has to affect the dynamic spine.

I'm shooting a measured 32 7/8" draw at 75lbs. I'm currently shooting .250 spine Victory RIP TKO Elites with 200gr on the front with insert and point, but also have some Apollo 250's as well. I shot GT Kinetic XT 200's for a few years. I didn't find them very durable at all. I had several break behind the insert. If it were me I would look at something more like the Sirius Apollo's, especially if you go .200 spine.
 
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I would expect it to. Consider a guy with a 29" draw shooting 30" arrows versus a guy with a 33" draw shooting 30" arrows........all else equal. That's a 40fps difference in arrow speed, therefore more energy launching that arrow. That has to affect spine.

The power stroke is 4" longer, so the duration is longer, giving it more speed. A bow that shoots an arrow 20 fps faster at the same draw length is pushing it a lot harder and faster. Now, same draw length and 10fps faster, that is pushing it harder. But gaining speed by lengthing it doesn't change the push.


I might not be 100% correct, I always assumed draw length played into it a lot, but in my experience it doesn't matter that much. The arrow is faster because it's on the string longer, not that it's getting pushed harder.
 

5MilesBack

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The arrow is faster because it's on the string longer, not that it's getting pushed harder.
How does being on the string longer make it faster? Energy has to come from somewhere......and go somewhere at the shot. I'm thinking it's that energy transfer that makes it faster.
 
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How does being on the string longer make it faster? Energy has to come from somewhere......and go somewhere at the shot. I'm thinking it's that energy transfer that makes it faster.
Because you don't use all the energy.
The limbs are only capable of a max peak.
70 lbs is 70 lbs.
Length gives more time for the available every to transfer.
Just like a short vs long barrel rifle. The cartridge is the same. Fps is more Because your transferring more energy.
 

TX_hunter

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Draw length absolutely affects arrow spine. A longer draw length puts more force on the arrow. The charts are a starting point, but if you run the numbers through any spine software, you will see the difference with a short vs long draw, leaving arrow length the same.
 

Marble

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@Billy Goat it would be interesting to see a side by side comparison between two bows, same IBO and poundage, but at like a 29 and a 33/32" draw length.

It makes sense to me how you described it. The force is still the same at short or long draw, just applied for different durations.

Maybe the longer duration is able to push on the arrow longer and thus able to make it flex more? Idk.. be interesting.

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Draw length absolutely affects arrow spine. A longer draw length puts more force on the arrow. The charts are a starting point, but if you run the numbers through any spine software, you will see the difference with a short vs long draw, leaving arrow length the same.

This is correct. Arrow spine is based on a 28” length. As arrow gets longer it gets weaker.

Black Eagle Rampage 150s at full length 32” will be closer to 200 spine. It may be your only option. I’d put the standard Easton 16gr AL hit inserts in and a get a field point test pack.


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TX_hunter

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@Billy Goat it would be interesting to see a side by side comparison between two bows, same IBO and poundage, but at like a 29 and a 33/32" draw length.

It makes sense to me how you described it. The force is still the same at short or long draw, just applied for different durations.

Maybe the longer duration is able to push on the arrow longer and thus able to make it flex more? Idk.. be interesting.

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Longer draw does result in more work done to the arrow. Work = force x distance. The result is more arrow speed at longer draw length.

Watch any bow review from Inside Out Precision on youtube. He tests all bows at 30" and 28", same arrows
 

TX_hunter

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Hey guys

Shooting a Mathews atlas at 74 lbs, 33.5 inch draw length (I’m 6’8” with 7ft wing span)

Looking at kinetic khaos 250 or 200?? Arrows will be uncut at 32in

The arrow charts only look at arrow length and poundage, does having a longer draw length than arrow length have an impact on when choosing a spine? Thanks!
The calculation showing you on the stiff side is with 24 grain insert and 125 grain point.

The one right in the middle is 100 grain insert with 125 grain point.
Screenshot_20220403-120226_qSpine.jpgScreenshot_20220403-120209_qSpine.jpg
 
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How does being on the string longer make it faster? Energy has to come from somewhere......and go somewhere at the shot. I'm thinking it's that energy transfer that makes it faster.

More energy transfer because it's on the string longer. It can't transfer a higher amount of energy without being higher poundage. However with a longer draw it has more time to transfer that energy, or a longer duration.
 

Bump79

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It's a two fold affect on spine but I think the major factor is arrow length. Think of a beam spanning a larger distance you need a stiffer beam to flex the same amount as a less stiff beam spanning a shorter distance.

@Billy Goat @5MilesBack
A bows kinetic energy is relative to the area underneath the curve on a draw force curve. More easier said KE equals work which is Force x Distance or poundage x (draw length - brace height). The poundage varying at each point in the draw on a compound.

KE = 1/2 Mass x Velocity squared. Reconfigure the equation and you get Velocity = sqrt (2 x KE / Mass). With that you'll see if you increase KE by lengthening your draw you will increase Velocity.

This might be a poor example but I'll give it a shot. Imagine a 300 hp electric car traveling 1/8 of a mile vs the same car going 1/4 mile. Same power but it will be going faster when it passes the 1/4 mile than it was the 1/8.
 
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Longer draw does result in more work done to the arrow. Work = force x distance. The result is more arrow speed at longer draw length.

Watch any bow review from Inside Out Precision on youtube. He tests all bows at 30" and 28", same arrows

The higher speed is the result of more work being done. But at 70#, with a 7" brace, 30" draw you have 23" of draw cycle. At 27" draw, 70#, 7inch brace you have 20" of draw cycle. Both bows aren't putting more than 70# of force on the back of the arrow. How fast that force ramps up from a standstill is important for the spine, the duration once its in motion isn't as important.


I'm not saying that draw length has no effect, but it's not nearly as much as I once thought. I have tuned enough stuff to determine in my eyes it's a lot more a function of total arrow length, front weight, total weight, and how hard or fast the cam ramps up. The total length of push isn't as important.


Just because it's longer, it doesn't put more force on the arrow. The force is 70#.
 

Bump79

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The higher speed is the result of more work being done. But at 70#, with a 7" brace, 30" draw you have 23" of draw cycle. At 27" draw, 70#, 7inch brace you have 20" of draw cycle. Both bows aren't putting more than 70# of force on the back of the arrow. How fast that force ramps up from a standstill is important for the spine, the duration once its in motion isn't as important.


I'm not saying that draw length has no effect, but it's not nearly as much as I once thought. I have tuned enough stuff to determine in my eyes it's a lot more a function of total arrow length, front weight, total weight, and how hard or fast the cam ramps up. The total length of push isn't as important.


Just because it's longer, it doesn't put more force on the arrow. The force is 70#.
Agreed. I'm not positive how much difference the KE actually effects spine but it doesn't seem to be nearly the factor that arrow length is.
 

TX_hunter

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The higher speed is the result of more work being done. But at 70#, with a 7" brace, 30" draw you have 23" of draw cycle. At 27" draw, 70#, 7inch brace you have 20" of draw cycle. Both bows aren't putting more than 70# of force on the back of the arrow. How fast that force ramps up from a standstill is important for the spine, the duration once its in motion isn't as important.


I'm not saying that draw length has no effect, but it's not nearly as much as I once thought. I have tuned enough stuff to determine in my eyes it's a lot more a function of total arrow length, front weight, total weight, and how hard or fast the cam ramps up. The total length of push isn't as important.


Just because it's longer, it doesn't put more force on the arrow. The force is 70#.
It would vary some from bow to bow, but there is some intersection where you can decrease draw length, add poundage, and the area under the draw force curve would be equal.

From some basic calculations, 3" of draw length is roughly equal to 10lbs of draw weight.
 

Marble

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Longer draw does result in more work done to the arrow. Work = force x distance. The result is more arrow speed at longer draw length.

Watch any bow review from Inside Out Precision on youtube. He tests all bows at 30" and 28", same arrows
That all makes sense to me. I just want to see one of the archery programs show me the difference just so I have a ball park of what the affect on draw length and arrow length is.

Just out of curiosity, 70 lbs bow, 28"and 32"draw lengths. Same arrow length for both at say 30.5. Will the same spine work for both?

This is just interesting to me. I have a 29-29.5" draw length depending on the bow. So this doesn't affect me. I'm just curious.

Edit
Actually nevermind... im reading above and it seems my thought process was right on with you guys. I just had a hard time putting it into words.

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So here's my point if I can explain it correctly:

What matters is say the first 3-5". I'm not certain where it is exactly, but it's the curve of "0" at full draw til it hits max draw weight. The faster that happens, the harder it pushes the back of that arrow. Once that arrow is moving, the added energy is from it being on the string, but it's not pushing it nearly as hard as that initial take off. So an additional 3" isn't as important, the arrow is already moving and from the spine standpoint all it knows is the original length of flex, with the weight pushing the back, and the weight on the front that initially wants to stay at rest. That longer time on the string allows more time for energy transfer, but it's not increasing the push on the arrow, at least it's pretty miniscule.

I
 
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Jg48

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Wow this has been a great response! Both on which spine arrow I should use and on whether draw length affects spine. Thanks everyone!

@TX_hunter thanks for the comparison charts. I am indeed running a 125gr point and the accutough insert that comes with the arrow is 24gr with the ballistic collar at 15.5gr. I have been getting a slight nock right tear when paper tuning and was wondering if it could be from spine being to stiff….Looks like I need to add some weight to the front of my arrow. Thanks!
 
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